Conventionally known methods for producing a polyester block copolymer include reaction of an aromatic polyester and a lactone; e.g., a method (Japanese Patent Application Laid-Open (kokai) No. 48-4116) including reaction of a crystalline aromatic polyester and a lactone; a method (Japanese Patent Application Laid-Open (kokai) No. 48-4115) including reacting a crystalline aromatic polyester with a lactone, to thereby yield an initial copolymer, and reacting a poly-functional acylating agent with the initial copolymer for chain-extension; and a method (Japanese Patent Publication (kokoku) No. 52-49037) including solid-polymerization of a lactone in the presence of a crystalline aromatic polyester.
Polyester block copolymers produced through these methods have excellent rubber-like elasticity and weatherability. However, these copolymers have drawbacks; i.e., poor heat resistance which induces a considerable drop in viscosity, strength, elongation, etc. due to exposure to high-temperature conditions for a long period of time. Furthermore, these copolymers have no strain hardenability, which is an important factor during blow molding, and, therefore, cannot provide molded products of uniform thickness through blow molding.
In order to improve heat resistance and moldability of the aforementioned types of polyester block copolymers, several methods have been proposed; e.g., a method (Japanese Patent Application Laid-Open (kokai) No. 58-162654) including addition of an epoxy compound having one or more functionalities; a method (Japanese Patent Application Laid-Open (kokai) No. 59-152947) including addition of an epoxy compound having one or more functionalities and an aliphatic carboxylic acid metal salt; and a method (Japanese Patent Application Laid-Open (kokai) No. 59-155458) including addition of an epoxy compound having one or more functionalities and an ethylene-carboxylic acid copolymer. However, compositions obtained through these methods have drawbacks; e.g., a drop in heat resistance, and relatively low melt viscosity and poor stability in quality caused by difficulty in correlating the amount of aliphatic carboxylic acid metal salt added and the dependency of melt viscosity on elongation strain rate.
Other proposed methods include a method (Japanese Patent Application Laid-Open (kokai) No. 60-170660) including addition of an epoxy compound and a metallic compound; a method (Japanese Patent Application Laid-Open (kokai) No. 4-178453) including addition of an epoxy compound having one or more functionalities and a sulfonate salt or a sulfate salt; and a method (Japanese Patent Application Laid-Open (kokai) No. 4-253764) including addition of an epoxy compound having on or more functionalities and a pentavalent phosphorus compound. However, these methods also have drawbacks; e.g., a large increase in viscosity or excessively small melt viscosity during molding. A method (Japanese Patent Application Laid-Open (kokai) No. 4-264156) including addition of an epoxy compound having one or more functionalities and a phosphite compound also has a drawback in that water resistance of products drops considerably.
Recently, in order to overcome all these drawbacks, a method (Japanese Patent Application Laid-Open (kokai) No. 07-331046) including addition of an epoxy compound having two or more functionalities and an imidazole compound has been proposed. However, this method also has a drawback, in that the dependency of melt viscosity on elongation strain rate (i.e., strain hardenability) is still poor and quality of blow-molded products varies. “Strain hardenability” is a property that melt viscosity increases with increasing elongation rate. Thus, when strain hardenability is large, a well-stretched (upon blow molding) portion is not further stretched by virtue of high viscosity, whereas when an insufficiently stretched portion is stretched by virtue of low viscosity, thereby resulting in uniform thickness.
In addition, similar methods (Japanese Patent Application Laid-Open (kokai) Nos. 10-25401, 10-30053, 11-21436, and 2000-143950) employing a low-volatile epoxy compound having two functionalities have been proposed. However, these methods have not yet resolved the problem that a large increase in viscosity during molding results in failure to stabilize quality of products.
The present invention has been accomplished in view of the aforementioned problems. Thus, an object of the present invention is to provide a polyester block copolymer composition which has such excellent moldability that the composition can be molded without problems through a variety of molding methods including blow molding and which is endowed with rubber-like elasticity and excellent heat resistance, water resistance, impact resistance, and flame retardancy.